Since the first report of bismuth germanium oxide (Bi4GeaO12 or BGO) as a scintillator material in 1973, this material has slowly started to replace the ubiquitous thallium-doped sodium iodide (NaI:Tl) in many of its applications. See W. M. Yen et al., Phosphor Handbook, 2nd ed.; CRC Press, Taylor & Francis Group: Boca Raton (2007); and E. Dieguez et al, J. Phys. C., Solid State Phys. 18, 4777 (1985). Some of the properties that have led to this substitution include the good stopping power of the high Z numbered Bi, high density (7.112 g/cm3), robust nature (hardness 5 Mho), air stability (non-hygroscopic), radiation hardness (stable to 5.1×104 Gy), small afterglow (0.005% after 3 ms), rapid rise/decay time (high scintillation efficiency), good energy resolution (5-20 MeV), low self-absorption, photofraction, and a four times larger absorption coefficient. See W. M. Yen et al., Phosphor Handbook, 2nd ed.; CRC Press, Taylor & Francis Group: Boca Raton (2007); E. Dieguez et al., J. Phys. C., Solid State Phys. 18, 4777 (1985); C. W. E. van Eijk, Nucl. Instr. Method. Phys. Res. A 460, 1 (2001); and G. C. Santana et al., J. Mater. Sci. 42, 2231 (2007). Even though BGO has only 20% of the light emission of NaI:Tl, the maximum occurs at 480 nm and covers a wide range of wavelengths (375 to 650 nm), which leaves a significant amount of the emission above 500 nm. Although this may not be favorable for scintillation collection by photomultiplier tubes, this broad emission can be effectively collected by new avalanche photodiode arrays. Furthermore, BGO produces ˜8500 photons per 1 MeV of high energy radiation that it absorbs. See W. M. Yen et al., Phosphor Handbook, 2nd ed.; CRC Press, Taylor & Francis Group: Boca Raton (2007); E. Dieguez et al., J. Phys. C., Solid State Phys. 18, 4777 (1985); and C. W. E. van Eijk, Nucl. Instr. Method. Phys. Res. A 460, 1 (2001). Combined, these properties make BGO interesting for scintillator applications that require detection of low amounts of γ-emitting material.
While there are numerous phase diagrams reported for BGO, four main phases are reported: sillenite (Bi12GeO20), aurivillius (Bi2GeO5), eulytite (Bi4Ge3O12), and benitoite (Bi2Ge3O9). See V. V. Zyryanov et al., Inorganic Materials 41, 618 (2005). Of these, the eulytite (E-BGO) phase is desired for scintillator applications based on its inherent properties. Since BGO has a cubic structure and is optically isotropic, polycrystalline ceramic components will not demonstrate light scattering at the grain boundaries. See W. M. Yen et al., Phosphor Handbook, 2nd ed.; CRC Press, Taylor & Francis Group: Boca Raton (2007). Thus, the production of E-BGO polycrystalline materials offers an attractive alternative to the conventional, laborious single crystal BGO scintillators. In addition, the smaller the size of the E-BGO particles, the higher the surface area is expected, which can significantly reduce the sintering temperature and promote the densification process. Combined, these properties can inhibit the formation of oxygen vacancies that can otherwise degrade the optical quality of BGO components. See A. F. Shimanskii and M. N. Vasil'eva, Refractories and Industrial Ceramics 42, 20 (2001); and M. G. Kisteneva et al., Russian Physics Journal 55, 444 (2012). Further, the synthesis of phase-pure E-BGO powders can replace non-stoichiometric BGO starting powders, thereby leading to higher quality, single crystal optical components and facilitating the fabrication of low cost, high volume BGO ceramic scintillators and/or large size γ-ray detectors. See I. I. Novoselov et al., Inorganic Materials 49, 412 (2013).
Several routes have been reported for the production of E-BGO. Typically, these methods follow the established Czochralski process to grow large single crystals of BGO in cube or cylindrical forms. See W. M. Yen et al., Phosphor Handbook, 2nd ed.; CRC Press, Taylor & Francis Group: Boca Raton (2007) and E. Dieguez et al., J. Phys. C., Solid State Phys. 18, 4777 (1985). Otherwise, synthetic routes to BGO are limited to melts, mechanochemical, and solution routes. See I. I. Novoselov et al., Inorganic Materials 49, 412 (2013); M. Ishii and M. Kobayashi, Prog. Cryst. Growth Charact. 23, 245 (1991); M. T. Borowiec et al., Proc. of SPIE 5136, 26 (2003); G. C. Santana et al., J. Mater. Sci. 42, 2231 (2007); V. V. Zyryanov et al., Inorganic Materials 41, 618 (2005); Z. S. Macedo and A. C. Hernandes, J. Am. Ceram. Soc. 85, 1870 (2002); Z. S. Macedo et al., NIM B 218, 153 (2003); T.-K. Tseng, University of Florida (2010); and D. E. Kozhbakhteeva and N. I. Leonyuk, J. Optoelectronics and Adv. Materials 5, 621 (2003). There are reports of the solid-state synthesis of the metal oxides that successfully generated nano-BGO:Eu. The two solution routes reported were of interest since they are a convenient method for production of large scale powders and have reportedly demonstrated control over the phase and morphology based on the choice of surfactants and solution pH. See S. Polosan et al., Optoelectronics Adv. Mater. 4, 1503 (2010). Both routes employ nitric acid (HNO3) to minimize bismuth reduction following solvothermal (SOLVO) and solution precipitation (SPPT) methodologies. See D. E. Kozhbakhteeva and N. I. Leonyuk, J. Optoelectronics and Adv. Materials 5, 621 (2003); and T.-K. Tseng, University of Florida (2010). Therefore, both methods require the addition of HNO3 and other reagents that can lead to trace impurities. The SOLVO route yields crystals up to 500 nm in size from a mixture of the metal oxides (Bi2O3 and GeO2) that are slurried in aqueous ammonium fluoride (NH4F) in the presence of HNO3 and heated at 310° C. at 20-50 MPa. See D. E. Kozhbakhteeva and N. I. Leonyuk, J. Optoelectronics and Adv. Materials 5, 621 (2003). In contrast, the SPPT route involves the dissolution of GeO2 and Bi(NO3)3 in HNO3, dilution with urea and water, and heating the reaction mixture to 90° C., followed by the addition of sodium hydroxide. The initial (2 min) material isolated from this complex reaction mixture was found to be amorphous, but heating for 1 hour led to phase-pure BGO. After decanting and heating to 500° C. flower-like BOO crystals were isolated with a trace of the sillenite impurity reported. See T.-K. Tseng, University of Florida (2010).
However, a need remains for a simple method to synthesize phase-pure E-BGO powders following a solution route that does not require additional thermal treatment.